Projectors for laser display can generally be categorized into one of three groups including graphics projectors, beam projectors, and audience scanning projectors.
Graphics projectors are typically those which project logos, text and other figures onto some projection surface such as a screen. To create images, these projectors employ an X-Y scanning system, usually including two small mirrors mounted on galvanometer scanners. One mirror scans the beam in one linear direction (for example, horizontally) onto the second mirror, which scans the beam in the perpendicular direction (for example, vertically). The combined X-Y motion is normally used to draw outline-type vector images, using a point-by-point “connect the dots” styled method, according to software commands from a programmable controller operably connected with the laser projector. The audience views these figures on the screen in the same way that an audience would view a movie being projected onto a screen.
A beam projector produces beams of light that are projected into mid-air. The beams are viewable in mid-air by virtue of fog, dust and moisture that either exists in the air or which is created by a performer or venue. The beams are often animated to produce a dynamic effect. The beams can be moved and animated in a number of ways. As described by way of example for embodiments of the invention, an X-Y scanning system is also used. The scanning system may be identical to that of graphics projectors (the projector is merely aimed into the air instead of at a screen), or the scanning system may scan more slowly than that of graphics projectors (since complex images may not be required). Use of an X-Y scanning system allows flexibility to create both simple placement of the beam to hit target mirrors or objects and also to allow more complex patterns such as circles and shapes to be projected.
With both graphics and beam projectors, the generated light, typically from a laser beam, does not come in contact with the audience. The light merely travels from the projector to its destination surface (in the case of graphics projectors), or along an uninterrupted path in mid air, and typically exclusively above the heads of the viewing audience (in the case of beam projectors).
Audience scanning projectors typically combine features of both graphics and beam projectors. Audience scanning projectors typically use X-Y scanners to project geometric figures, patterns and arrays of light beams directly into a viewing audience. As with beam projectors, when the laser is projected toward an audience, its beam also illuminates any fog, dust, and moisture in the air. The beams may create dancing sculptures that are very pleasing to audience members and the beam generally comes in direct contact with the audience. The generated effect creates an illusion of being surrounded by a tunnel of light and by other geometric shapes that are formed by the light. Such effects have been compared to being inside a fireworks display, or being at the bottom of a swimming pool filled with light. By way of example, one audience scanning projector, as known in the art, is described in the above referenced U.S. utility patent application Ser No. 11/814,310 for “Laser Projector for Audience Scanning” and herein incorporated by reference.
Audience scanning projectors are often placed at a height of 3 meters, which is above the heads of all audience members. This is because the typical program material (image file data and abstracts) projected by audience scanning projectors uses the concept of a “horizon” to create certain shapes. It is known by laser show programmers that audience members will not reside above the horizon, and thus, certain patterns, such as non-moving beams may be placed there which are known to be above the heads of those in the audience. Likewise, it is known that audience members will probably reside below the horizon, and therefore only sheets and cones and faster-moving patterns will typically be placed there.
In the case of each of the three projector types described above, the X-Y signals and beam power level signals are generated by a programmable controller which generally comprises a personal computer having suitable interface hardware, and running software for generating the images, patterns and shapes. The hardware generally includes an interface circuit board that connects to the computer. This interface circuit board includes digital-to-analog converters and voltage amplifiers, so that signals can be produced which correspond to X-Y beam positions, and to beam power levels. The X-Y beam positions and beam power levels produced by the interface hardware are sometimes referred to as “command signals,” since these signals represent the software's intention for the projector to follow. The software program generates the X-Y beam positions and beam power level “command signals” and periodically transfers these as digital data to the digital-to-analog converters in the interface circuit board. Those skilled in the art know that any suitable interface hardware and software may be used to control any of the three projector types mentioned above. However, by way of example for embodiments of the present invention, hardware and software systems include the QuadMod™ series of hardware boards and Lasershow Designer™ series of laser software, both from Pangolin Laser Systems, Orlando, Fla.
When projecting a laser beam toward a viewer, eye safety is a major concern. If an intense laser beam were to stop scanning and stopped directly on the pupil of a viewer's eye, retinal damage can occur if the beam has sufficiently high power and a sufficiently long dwell time. Likewise, even if the beam is not stopped but is scanned across the pupil of an eye, it can still cause retinal damage if the beam power is high enough, or if the beam is scanning slowly enough.
In audience scanning projectors in the current state of the art, the X and Y beam position signals generated by the X-Y scanners are mathematically differentiated to produce an output equivalent to X and Y beam velocity. The X and Y beam velocities are added together to produce the total beam velocity. This total beam velocity is monitored and compared to some pre-set minimum allowable velocity to make sure that the beam velocity is sufficiently high. If the beam were to stop scanning, producing zero velocity, or if the velocity were to otherwise drop below some preset threshold, this would be considered a “scanning failure.” Under a scanning failure condition, the beam may be completely turned off by the light beam modulator or by a shutter. This type of system is called a “scan-fail monitor”. Note that a scan-fail monitor is most often implemented in the form of analog signal conditioning components, but may also be implemented with computer hardware and software. By way of further example, and with reference again to U.S. utility patent application Ser. No. 11/814,310 for “Laser Projector for Audience Scanning,” a scan-fail monitor as know in the art is described.
While scan-fail monitors provide some level of protection for the audience, there are a number of problems that still remain. First, a scan-fail monitor does not provide automatic power level control in different regions of the scan field. For example, scan-fail monitors are not capable of allowing a higher power level over the audience's heads or below their eyes. Second, scan-fail monitors can be easily “fooled” into believing that there is a safe condition when there is not, because they typically only monitor the rate of change of position and do not track the actual position of the beam. For example, if the beam alternates between two fixed locations, thereby concentrating 50% of the beam power in each position, the scan-fail monitor may allow this condition since the beam is technically scanning. However, in many instances, a 50% concentration of beam power could be hazardous. Therefore, improvements are still needed over the use of a scan-fail monitor alone.
Referring again to the above referenced audience scanning projector of the current state of the art, a laser generates a beam of light which is modulated. The modulation is performed either directly by the laser power supply, or externally by a modulator such as an acousto-optic modulator. After being modulated, the beam is directed to X-Y vector scanners and then projected directly into the audience. A scan-fail monitor may be used to detect if the scanning has stopped, or slowed to an unacceptably low level. Aside from the laser, modulator, shutter and X-Y scanners, typically no additional optical elements are used.
In this configuration, it could be said that the “raw laser beam” is used directly to illuminate audience members. However, a typical laser used for laser display applications has a beam diameter of 2 millimeters and divergence of around 1 milliradian. With this small size beam and low divergence, it is very possible that, during the scanning action, the entire laser beam will be smaller than 7 millimeters (which is the internationally-agreed-upon pupil diameter used for the purposes of safety evaluation) within the audience. Because the laser beam diameter is typically so small in the audience, it means that no greater than around 5 milliwatts of laser power can be used to create the laser display, regardless of the sophistication of the scan-fail monitor. If a higher power is used, the display will not be able to meet Class 1 standards for laser safety.
Although it is not typically done, it is known to use a lens within the beam path of an audience scanning laser projector, such as a lens whose power has approximately a −1.0 diopter. The lens may be placed between the modulated laser beam and the X-Y scanners, or immediately at the exit of the scanners. A lens whose power is negative will increase the divergence of the laser beam so that the beam diameter will be greater than 7 millimeters within the audience. When the beam is greater than 7 millimeters, the entire beam will no longer fit through the pupil of a viewer's eye and thus, not all of the laser beam's power will be delivered to the retina if the laser beam does land on someone's eye.
Light irradiance (which is the power per unit area) is governed by the “inverse square law”, which states that, for a given beam power, if you double the beam diameter, the power-per-unit-area will be decreased by a factor of four. Thus the power-per-unit area decreases by the inverse-square of beam diameter. Since the irradiance of the laser beam has the greatest implication to laser safety, it can be shown that decreasing the irradiance by increasing divergence is one effective means of increasing the safety of audience scanning projectors.
By using a lens with negative optical power to increase the divergence, this will decrease the power-per-unit area (irradiance) of the laser beam, thus making it possible to use a laser with higher than 5 milliwatts laser power, and still meet Class 1 standards for laser safety. In fact, by using a lens whose power is −1.0 diopters, the laser beam power can be increased to around 250 milliwatts for a typical size audience and a typical distance from the laser projector.
The increased divergence allows for a higher power laser to be used—thus, this generally provides a much more dramatic and stunning laser light show. However, this approach is also not without problems. Placing a lens with negative optical power before or after the scanners will decrease the divergence in all parts of the scan field, including above the heads of audience members. As stated above, laser projectors may be used for mirror targeting applications where the laser beam is directed to a mirror, to create a static beam sculpture. Typically a small beam is used for mirror targeting. The larger beam diameter caused by the lens makes such beam targeting virtually impossible. Another drawback is that, as the beam diameter is increased, the projected effect looks more cloudy and foggy, whereas the raw beam projected from the laser makes it look as though the light is cutting through the air like a knife.
It is also useful to have the lens located on a mounting arrangement external to the projector, so that if needed, the lens can be changed. However, an external mounting arrangement can also be tampered with. For example, if the laser projector is installed in a disco or night club, a rogue club operator or DJ might be tempted to completely remove the lens, thus allowing more intense and low-divergence beams to be projected everywhere, including into the audience. However, as noted above, low divergence beams projected into the audience would be potentially hazardous, especially if the power of the laser is greater than 5 milliwatts.